In order to injection mold large articles having thin walls, it is well known that the injection phase of the process must be accomplished very quickly in order to fill the mold cavity completely before the resin has had time to cool sufficiently to impede filling. Methods of doing this have included precompression molding, multiple gates in the same mold cavity or using a single large diameter valve gate.
Precompression molding uses a shut off nozzle valve which remains closed until the resin in the injection unit had been compressed to the full injection pressure, typically 20,000-40,000 psi. Then the nozzle is pressed against the mold sprue bushing and automatically opens to allow the precompressed resin to literally explode into the empty cavity, filling it almost instantly. This process has several disadvantages which include rapid shear heating of the resin as it flows through the comparatively small gate in the cavity. This causes resin degradation. A second disadvantage is the difficulty in trying to vent the air in the mold cavity quickly enough to prevent "dieseling", that is burning of the melt front by the rapidly heated air. Increasing the vents to solve this problem increases the possibility of flashing the mold. Alternatively, relaxing the mold clamping force, even temporarily in the cycle, to allow the mold to "breathe" causes core shifting and alignment problems.
Multiple gates feeding the same mold cavity incur the additional expense of providing the nozzles and their associated hot runner manifold for supplying them. This method also introduces several simultaneous melt fronts into the cavity resulting in flow marks and the possibility of trapping gas in the mold between the melt fronts. U.S. Pat. No. 5,013,513 to Schad et al. is an example of multiple gating a thin wall part.
Large diameter mechanical valve gates offer the possibility of presenting a large opening in the mold cavity to allow rapid filling without degrading the resin. However, large diameter valve gates have not been entirely satisfactory. Copending U.S. patent applications Ser. Nos. 707,660 and 707,666 both abandoned, filed May 30, 1991, show such arrangements. The inherent problem with this style of gate is the conflicting requirement to both cool the valve's molding surface while heating the valve stem. By adding cooling and heating provisions in the valve stem, the structural strength of the stem is considerably weakened, risking collapse under injection and holding pressures which must be resisted when the valve is near closing.
U.S. Pat. No. 4,808,106 to von Holdt shows a "Flex Gate". This large diameter valve gate has no moving parts in the accepted sense, however the valve action is caused by injection pressure bending the valve structure during injection. The removal of melt pressure allows the valve to close as the elastic valve structure resumes its former undeflected state. The inherent problems of heating and cooling the valve stem are also present in this configuration, coupled with the comparative stiffness of the valve. Considerable melt pressure must be built up prior to the valve's opening, while closing may occur at such a high pressure that maintaining a hold pressure on the melt in the cavity as the resin cools may prove detrimental. The intermittent contact between the valve stem and the nozzle body will cause wear to occur, so even this "non-moving" design will suffer wear.
It is well known in the art that "flash" gating or fan gating using a wide edge gate in conventional cold runner molding provides a comparatively large opening for the rapid filling of parts. After the part has cooled and the mold opens, the attached runner and gate are removed leaving the usual gating mark on the edge of the part.
The principle of hot tip or hot edge nozzle is also well known. The constantly heated nozzle tip maintains the resin melted behind a frozen "plug" of resin in the gate. The application of injection pressure to the melted resin in the nozzle channel causes the frozen plug to be blown in to the mold cavity effectively opening the gate to permit the melted resin to fill the cavity. After the flow of resin stops, when the mold cavity is filled, the cooled gate area freezes the plastic locally into a "plug" to prevent melt drooling into the cavity, while the heated nozzle maintains the resin in the nozzle channel molten. The cycle is then repeated. These types of nozzles have no moving parts and to date have been limited to small gate diameters, typically 0.020"-0.080" in order to permit rapid freezing of the plug. Larger diameter gates which provide large openings for rapid mold filling have not been successfully constructed using this technique to date, especially when molding thin sectioned parts.
It is therefore a principal object of the present invention to overcome the foregoing disadvantages and to provide a large opening for rapid filling of a mold cavity in injection molding while at the same time providing the necessary heating and cooling provisions in the injection nozzle and in the inlet conduit or gate leading to the mold cavity.
It is a further object of the present invention to provide an apparatus as aforesaid which may achieve these advantages with an injection nozzle having no moving parts.
Further objects and advantages of the present invention will appear hereinafter.